This invention relates to a combustion chamber for a direct injected engine and more particularly to an improved combustion chamber configuration and method of operating a direct injected engine.
In the interest of providing further enhancements for the performance and emission control and fuel economy of engines, direct cylinder injection is being applied. One of the advantages of direct cylinder injection is that it permits the charge in the combustion chamber to be stratified. If stratification can be achieved, then the engine need not be filled with a homogeneous mixture under lighter load and running conditions. This will reduce the amount of unburned hydrocarbons in the exhaust gases as well as the fuel economy.
However, it is quite difficult to obtain effective stratification in open chamber engines. This is particularly true since the injected fuel tends to have somewhat larger particles and these larger diameter particles take time to evaporate before the fuel is in a condition where it can be effectively burned. This is particularly true under low speed conditions when only a small amount of fuel is injected.
There have been proposed types of combustion chambers where the piston is provided with a bowl in its head and this bowl is offset toward one side of the combustion chamber, generally the intake side, so as to permit a central spark plug position and to encourage the flow of fuel from the bowl into the area where the spark gap exists.
FIG. 1 is a view of a type of prior art construction that is designed to utilize this principle. As seen in this figure, the engine includes a cylinder block 11 in which a cylinder bore 12 is formed. A cylinder head 13 is affixed to the cylinder block and closes the upper end of the cylinder bore 12. A piston 14 reciprocates in the cylinder bore 12 and defames with it and with a recess 15 in the lower surface of the cylinder head 13 a combustion chamber.
The piston 14 is formed with a domed head having a raised portion 16. An offset recess or bowl 17 is formed in one side of the piston 14 and this is the side where the intake passage or passages 18 enter the combustion chamber through valve seats 19.
A spark plug 21 is positioned centrally in the cylinder bore 12 and at the peripheral edge of the bowl 17.
A fuel injector 22 is mounted in the cylinder head 13 and is designed so as to spray in a pattern as shown over a relatively wide arc and toward the base of the bowl 17. Theoretically this causes the fuel to deposit and move as shown by the white arrow in this figure toward the gap of the spark plug 21 so as to ensure the presence of a stoichiometric mixture even under light loads at the time the spark plug 21 is fired.
To obtain the effects desired, it is relatively necessary to inject the fuel substantially late in the cycle and when the piston is approaching its top dead center position before the spark plug 21 is fired. This means that the fuel that is injected does not remain in the cylinder for a very long period of time before the spark plug 21 fires. Thus, there may be substantial unvaporized fuel which will still result in some hydrocarbon emissions.
It is, therefore, a principal object of this invention to provide an improved combustion chamber configuration and operational method wherein the fuel can be injected earlier into the cycle and still will remain in a stratified condition.
It is a further object of this invention to provide an improved combustion chamber configuration and injection pattern that permits the fuel to more fully vaporize before the spark plug is fired, even under low speed/low load conditions.
Of course, the maximum time that the fuel can be maintained in the combustion chamber is somewhat limited by the time that the exhaust valve is opened. If fuel is injected at a time when the exhaust valves are open, then some of this fuel may pass out of the combustion chamber with exhaust gases. This is obviously undesirable.
It is, therefore, a still further object of this invention to provide an improved combustion chamber configuration and injection strategy wherein the fuel can be injected at a time when the exhaust valve is open without escaping from the exhaust port.
By injecting fuel earlier in the cycle, greater time for vaporization will also occur. Also, it is desirable if some of the fuel can be moved into proximity with the exhaust valve or valves at a time when they are closed or substantially closed so that the heat of the valve can help in fuel vaporization.
It is, therefore, a still further object of this invention to provide an improved combustion chamber configuration and injection strategy wherein the heat of the exhaust valves can be utilized to assist in fuel vaporization without risk of the fuel passing out of the exhaust port.